The amino acid composition and tryptic peptide maps of three isozymes, LDH-M, LDH-H, and LDH-X, purified from mouse muscle, heart, and testis, respectively, has been compared. The testicular isozyme LDH-X has also been purified from both rat and hamster, and its amino acid composition determined in each case. Isozyme LDH-M has also been purified from horse muscle, and its enzymatic properties, amino acid composition and tryptic peptide map determined.